OPPORTUNITIES AND CHALLENGES: 40AR/39AR K-FELDSPAR THERMOCHRONOLOGY

Although ⁴⁰Ar/³⁹Ar K-feldspar MDD thermochronology is a mature method that has revealed excellent knowledge of geological histories, it remains a technique with challenges and opportunities. Ideally suited samples for bulk analysis are devoid of chemical and microstructural changes post argon closure and during laboratory argon extraction. The later is unlikely to occur and known changes remain debated as to limitations they may impose for extracting accurate argon kinetic parameters. General compatibility with other thermochronometers and geological constraints indicates that Arrhenius parameter inaccuracies related to step-heating induced changes are second-order compared to the first-order knowledge provided by continuous thermal history extraction. Also, sample stability following initial argon closure appears to be more common than its counterpart because, again, the MDD method often provides accurate thermal histories.

Advancing the future of the MDD method appears to lay in fully utilizing microbeam sample characterization coupled to high precision argon analysis on the same characterized K-feldspar fragment or spot. Recent increases in precision and sensitivity provided by new multi-collector mass spectrometers reveal new challenges towards understanding the details of the MDD method, but more importantly open new levels of opportunity. MDD analysis of tiny fragments of multiple generations of alkali-feldspars from within single rocks is the path forward towards fully exploiting the information embodied within in the argon isotopic system. Extracting both thermal and fluid histories are the challenges and the opportunities that are within reach because of new technologies. Examples of combined microbeam chemistry and argon extraction that provide new insight and also reveal complexities are provided by detailed work on the Klokken syenite, Chain of Ponds pluton and Shap granite. Incompatibilities of thermal histories from individual K-spar fragments are generally found to be caused by low temperature growth of 2^nd generation K-spar. Dating this generation provides new opportunities for understanding rock histories. Avoiding this generation when selecting material for MDD analysis minimizes method complexity and improves thermal history accuracy.